1The OSD Standard
2================
3OSD (Object-Based Storage Device) is a T10 SCSI command set that is designed
4to provide efficient operation of input/output logical units that manage the
5allocation, placement, and accessing of variable-size data-storage containers,
6called objects. Objects are intended to contain operating system and application
7constructs. Each object has associated attributes attached to it, which are
8integral part of the object and provide metadata about the object. The standard
9defines some common obligatory attributes, but user attributes can be added as
10needed.
11 12See: http://www.t10.org/ftp/t10/drafts/osd2/ for the latest draft for OSD 2
13or search the web for "OSD SCSI"
14 15OSD in the Linux Kernel
16=======================
17osd-initiator:
18 The main component of OSD in Kernel is the osd-initiator library. Its main
19user is intended to be the pNFS-over-objects layout driver, which uses objects
20as its back-end data storage. Other clients are the other osd parts listed below.
21 22osd-uld:
23 This is a SCSI ULD that registers for OSD type devices and provides a testing
24platform, both for the in-kernel initiator as well as connected targets. It
25currently has no useful user-mode API, though it could have if need be.
26 27exofs:
28 Is an OSD based Linux file system. It uses the osd-initiator and osd-uld,
29to export a usable file system for users.
30See Documentation/filesystems/exofs.txt for more details
31 32osd target:
33 There are no current plans for an OSD target implementation in kernel. For all
34needs, a user-mode target that is based on the scsi tgt target framework is
35available from Ohio Supercomputer Center (OSC) at:
36http://www.open-osd.org/bin/view/Main/OscOsdProject 37There are several other target implementations. See http://open-osd.org for more 38links.
39 40Files and Folders
41=================
42This is the complete list of files included in this work:
43include/scsi/
44 osd_initiator.h Main API for the initiator library
45 osd_types.h Common OSD types
46 osd_sec.h Security Manager API
47 osd_protocol.h Wire definitions of the OSD standard protocol
48 osd_attributes.h Wire definitions of OSD attributes
49 50drivers/scsi/osd/
51 osd_initiator.c OSD-Initiator library implementation
52 osd_uld.c The OSD scsi ULD
53 osd_ktest.{h,c} In-kernel test suite (called by osd_uld)
54 osd_debug.h Some printk macros
55 Makefile For both in-tree and out-of-tree compilation
56 Kconfig Enables inclusion of the different pieces
57 osd_test.c User-mode application to call the kernel tests
58 59The OSD-Initiator Library
60=========================
61osd_initiator is a low level implementation of an osd initiator encoder.
62But even though, it should be intuitive and easy to use. Perhaps over time an
63higher lever will form that automates some of the more common recipes.
64 65init/fini:
66- osd_dev_init() associates a scsi_device with an osd_dev structure
67 and initializes some global pools. This should be done once per scsi_device
68 (OSD LUN). The osd_dev structure is needed for calling osd_start_request().
69 70- osd_dev_fini() cleans up before a osd_dev/scsi_device destruction.
71 72OSD commands encoding, execution, and decoding of results:
73 74struct osd_request's is used to iteratively encode an OSD command and carry
75its state throughout execution. Each request goes through these stages:
76 77a. osd_start_request() allocates the request.
78 79b. Any of the osd_req_* methods is used to encode a request of the specified
80 type.
81 82c. osd_req_add_{get,set}_attr_* may be called to add get/set attributes to the
83 CDB. "List" or "Page" mode can be used exclusively. The attribute-list API
84 can be called multiple times on the same request. However, only one
85 attribute-page can be read, as mandated by the OSD standard.
86 87d. osd_finalize_request() computes offsets into the data-in and data-out buffers
88 and signs the request using the provided capability key and integrity-
89 check parameters.
90 91e. osd_execute_request() may be called to execute the request via the block
92 layer and wait for its completion. The request can be executed
93 asynchronously by calling the block layer API directly.
94 95f. After execution, osd_req_decode_sense() can be called to decode the request's
96 sense information.
97 98g. osd_req_decode_get_attr() may be called to retrieve osd_add_get_attr_list()
99 values.
100 101h. osd_end_request() must be called to deallocate the request and any resource
102 associated with it. Note that osd_end_request cleans up the request at any
103 stage and it must always be called after a successful osd_start_request().
104 105osd_request's structure:
106 107The OSD standard defines a complex structure of IO segments pointed to by
108members in the CDB. Up to 3 segments can be deployed in the IN-Buffer and up to
1094 in the OUT-Buffer. The ASCII illustration below depicts a secure-read with
110associated get+set of attributes-lists. Other combinations very on the same
111basic theme. From no-segments-used up to all-segments-used.
112 113|________OSD-CDB__________|
114| |
115|read_len (offset=0) -|---------\
116| | |
117|get_attrs_list_length | |
118|get_attrs_list_offset -|----\ |
119| | | |
120|retrieved_attrs_alloc_len| | |
121|retrieved_attrs_offset -|----|----|-\
122| | | | |
123|set_attrs_list_length | | | |
124|set_attrs_list_offset -|-\ | | |
125| | | | | |
126|in_data_integ_offset -|-|--|----|-|-\
127|out_data_integ_offset -|-|--|--\ | | |
128\_________________________/ | | | | | |
129 | | | | | |
130|_______OUT-BUFFER________| | | | | | |
131| Set attr list |</ | | | | |
132| | | | | | |
133|-------------------------| | | | | |
134| Get attr descriptors |<---/ | | | |
135| | | | | |
136|-------------------------| | | | |
137| Out-data integrity |<------/ | | |
138| | | | |
139\_________________________/ | | |
140 | | |
141|________IN-BUFFER________| | | |
142| In-Data read |<--------/ | |
143| | | |
144|-------------------------| | |
145| Get attr list |<----------/ |
146| | |
147|-------------------------| |
148| In-data integrity |<------------/
149| |
150\_________________________/
151 152A block device request can carry bidirectional payload by means of associating
153a bidi_read request with a main write-request. Each in/out request is described
154by a chain of BIOs associated with each request.
155The CDB is of a SCSI VARLEN CDB format, as described by OSD standard.
156The OSD standard also mandates alignment restrictions at start of each segment.
157 158In the code, in struct osd_request, there are two _osd_io_info structures to
159describe the IN/OUT buffers above, two BIOs for the data payload and up to five
160_osd_req_data_segment structures to hold the different segments allocation and
161information.
162 163Important: We have chosen to disregard the assumption that a BIO-chain (and
164the resulting sg-list) describes a linear memory buffer. Meaning only first and
165last scatter chain can be incomplete and all the middle chains are of PAGE_SIZE.
166For us, a scatter-gather-list, as its name implies and as used by the Networking
167layer, is to describe a vector of buffers that will be transferred to/from the
168wire. It works very well with current iSCSI transport. iSCSI is currently the
169only deployed OSD transport. In the future we anticipate SAS and FC attached OSD
170devices as well.
171 172The OSD Testing ULD
173===================
174TODO: More user-mode control on tests.
175 176Authors, Mailing list
177=====================
178Please communicate with us on any deployment of osd, whether using this code
179or not.
180 181Any problems, questions, bug reports, lonely OSD nights, please email:
182 OSD Dev List <osd-dev@open-osd.org>
183 184More up-to-date information can be found on:
185http://open-osd.org 186 187Boaz Harrosh <bharrosh@panasas.com>
188Benny Halevy <bhalevy@panasas.com>
189 190References
191==========
192Weber, R., "SCSI Object-Based Storage Device Commands",
193T10/1355-D ANSI/INCITS 400-2004,
194http://www.t10.org/ftp/t10/drafts/osd/osd-r10.pdf 195 196Weber, R., "SCSI Object-Based Storage Device Commands -2 (OSD-2)"
197T10/1729-D, Working Draft, rev. 3
198http://www.t10.org/ftp/t10/drafts/osd2/osd2r03.pdf 199